It is standard practice in the fabrication of an intermediate layer or cover layer of a golf ball to utilize an injection mold having two mold plates with hemispherical cavities that mate to form a spherical shape when the mold halves are joined. At the initial stage of the injection molding process, a golf ball core is supported centrally within the mold by a plurality of retractable pins near the upper and lower poles of the mold cavity so as to leave a space for forming an intermediate or cover layer about the core. A thermoplastic or thermosetting material then it injected into the mold cavity near the equator of the mold cavity. The retractable pins hold the core in place while the thermoplastic or thermosetting material injected into the mold cavity fills the void between the golf ball core and the mold. Once the void is nearly filled but before the injected material has completely hardened, the centering pins holding the core in place retract into pin holes and the injected material fills the voids left by the pins. The injected material then cools and hardens to form an intermediate layer or cover of the golf ball.
The retractable pins in conventional injection molding are tightly engaged with the golf ball core initially to center the core within the mold and subsequently to maintain the core's position during the injection molding process. For instance, injection of thermoplastic or thermosetting material into the mold cavity through a plurality of edge gates can cause significant uneven pressure of the core of the ball, particularly during the early stages of the injection process when the core is only minimally covered by injected material. The retractable pins counteract the unbalanced forces on the core to maintain the core's desired position. The core becomes held more securely in place by the injected material as the material fills the mold and surrounds the core and the pins. Once the core is sufficiently covered by injected material, the pins can be disengaged from the core while the injected material is still moldable and any remaining voids in the mold are filled by the injected material without affecting the position of the core. All trapped air and gasses evacuate the cavity via the retractable pins and vent pins located near the upper and lower poles of the mold cavity. Once the injected material has cooled sufficiently, the mold cavity is opened and the ball is ejected from the mold by striking the pins against the ball.
Use of a plurality of retractable pins to securely position the golf ball core during the injection process is known to cause wear at the interface between the surfaces of the pins and the surfaces of the pin holes in the mold plate through which the pins are inserted into the mold cavity. Typically, the face of the pins that contact the core of the ball are not normal, i.e. perpendicular, to the direction of the axial force applied to the pins to cause them to engage with the core. In a conventional retractable pin golf ball injection mold, illustrated in FIGS. 1 and 2, the pins are engaged with the core by being inserted into the mold in a vertical plane. The faces of the pins, however, are angled so that the they contact the core essentially along a tangent to the surface of the ball. For forming intermediate layers, the faces of pins that contact the ball typically have approximately a 20 degree angle cut, whereas the tips of pins in an injection mold to form a cover layer have an angle with dimple radius formed on the end thereof. The faces of the pins also may be machined to match the curvature of the spherical mold cavity.
Because the faces of the pins in contact with the ball are not normal to the direction of the vertical axial force applied to the pins engage them with the core, the core of the ball applies a counterbalancing force on the pins that has axial load and a cantilever load. As the pins move under this cantilever load when engaging or disengaging from the ball, the pin holes are worn out of round and the pins may experience extensive wear. In some instances, galling of the pin and pin hole may result. Wear between the pins and pin holes eventually becomes excessive and allows injected material to flow into the worn area, causing undesirable flash on the surface of the molded layer of the ball. The result of this undesired wear is that the manufactured balls require additional process steps to remove the flash and the mold must be shut down periodically for inspection, repair and/or replacement of worn tooling.
A second problem associated with the use of multiple pins to position the ball in the mold cavity is that injected material may engulf the pins before they are disengaged, thereby leaving voids of trapped air and other gasses in the locations where the pins contact the core of the ball. While the pin holes may be used to vent these trapped gasses, it is desirable to evacuate trapped air and gasses through vent pins located at or near the poles of the mold cavity. In addition, injected material that contacts the retractable pins immediately cools, which in turn slows the progression of covering material and causes the flow terminus to not meet at the poles of the ball where vent pins typically are located.
Another problem that results from use of multiple pins occurs during ejection of the ball from the mold. After the injected material has filled the mold cavity and has sufficiently hardened, the mold is opened and the ball is ejected from the mold by striking the pins against the surface of the ball. Because the faces of the pins comprise a small surface area, the impact force needed to eject the ball from the mold can impart high stress loads upon the molded layer. These high impact forces may weaken or cosmetically damage the molded layer, thereby diminishing the performance or durability of the ball, or cause the molded layer to become deformed, crack or tear.
Thus, it is desirable to substitute the multiple pins used in conventional golf ball injection molding with a device that permits proper positioning of the core without the associated problems of wear, having improved venting of trapped gasses, and reducing stresses imparted to the molded layer upon ejection from the mold.